In high resolution optical inspection imaging systems, it is important to ensure that an object plane of the optical system remains substantially in focus with respect to an image plane of the optical system. Doing so ensures that the resulting images are of sufficient quality for inspection purposes.
Ensuring that an optical system remains in focus can be accomplished in a number of ways. One way is to use an auto-focus technique in which a series of images at different focal positions are captured and then assessed to identify the optimal focal position. This approach is well understood, but is very slow as it requires multiple images at each position at which one desires to find a focal position.
Another technique is to use a height sensor such as a laser triangulation system or a capacitative height sensor to determine the height of a surface that is being imaged. Once the height has been determined, the optical system may be adjusted to ensure the object plane of the optical system is placed at the surface of the object being imaged. Such height sensors may be positioned to determine the height of the substrate within the field of view of the optical system or at a lateral offset from the field of view.
Systems that capture height at a lateral offset implement a mathematical transform to ensure that the height measured may be used at the field of view for focal positioning. It may be preferable to avoid the need for implementing such a transform and so it is often desirable to use a height sensor, as mentioned above, that determines the height of the object being imaged within the field of view. One such system uses a laser triangulation type sensor in which a measurement beam is incident on the object being imaged within the field of view. Given the extremely tight dimensional tolerances inherent in a high resolution imaging system, this type of system requires specially arranged objectives that have one or more optical components of the height sensor, either an optical element or a portion of an optical path, formed therein. The specialized nature of these systems reduces their flexibility, increases their expense, and makes it difficult to modify the system to capture images at different resolutions.
Another type of height sensor that measures surface height of an object being imaged at the field of view of an imaging system is adapted to operate within the actual optical path of the imaging system. By directing a measurement beam, such as the beam of a laser triangulation sensor, through a lens of the optical system onto the object being imaged within the field of view can provide an indication of the height of object. Such measurement systems require direct integration with an imaging system, which can be both complex and quite expensive. Further, light from a measurement beam can interfere with an imaging process.
Accordingly, there is a need to provide a focus height sensor that is inexpensive, simple to install and maintain, and which can measure the height of an object at a field of view of an imaging system.